Inductively working transponders have been known for a long time. They are employed in many cases as access control means e.g. on security doors. A so-called card reader is located in front of the door.
In order to enter the secured area, the user has to hold a small authorizing element, often in the shape of a cheque card, close to the card reader. Inductive power can be fed into the card due to its close proximity. Thereupon, the card itself inductively conveys a digital data word back to the card reader for identification purposes. The card reader compares the data word with data stored therein and operates the door opening system if there is an appropriate confirmation. In similar systems, the current data word on the card can also be changed e.g. modified by the card reader. One disadvantage of these systems is that the card has to be placed directly on the reader every time in order to e.g. open a door. This entails stopping one's progress, getting out the authorization card and holding the card in front of the card reader until the card reading device has released e.g. the entrance. Afterwards, the card has to be put back properly into its starting position, e.g. into a trouser pocket.
Transponders utilising radio transmission techniques would guarantee a greater range, but since it is difficult to direct radio waves and, on the other hand, they can also pass through walls and ceilings, the use of a radio transponder is not secure.
Moreover, the door would then be regularly opened quite unintentionally by authorized persons who are e.g. merely passing the door.
There is often a desire in hospitals for example, for one to be able to simply go towards a door which then opens with appropriate authorization whilst one is some distance away. If the transponder is accommodated in the frequently worn name tag of a person having authorized access, he will be able to push an e.g. patient's bed in front of himself and the door through which he has to go will open whilst he is still e.g. 5 m away. Consequently, his passage will not be interfered with and time can thereby be saved. In this situation, the ability to precisely direct optical beams of light has substantial advantages: only an authorized person going towards the door will activate it even from quite far away, whilst persons having the same access authorization but who are just passing by in the direct vicinity of the door will not activate it. The fixed device for the functioning of the transponder system can also be weather-protected behind a protective glass cover (a double glass sheet with a gap therebetween). A substantial time advantage also results in the case of persons who, for occupational reasons, must pass quickly through doors without having to take out their access authorization and stop at the reading device. This is often desired at airports since, there in particular, attention has to be paid to time-saving working patterns.
A further advantage results when using an optical transponder in the case of secured parking lots. The optical transponder can be mounted inside the vehicle behind the windscreen so that, when driving past an appropriate reader and upon its access authorization being recognized, the vehicle barrier is operated and the vehicle let in. Here, there is no need to decelerate and stop, hold out the authorization card to the reader and then drive off again.
Optical systems for access control purposes are of course known in the trade, they generally need manual release of a switch for activation purposes. Systems operating without manual activation have not been able to enter the market so far because of their size or the weight of the necessary batteries and the associated operating costs.
Transmitter units for optical signals in the infrared range are generally known. Remote controls, in particular for TV and the like, usually work with coded infrared light. These transmitter units are activated by depressing a key and they then send out coded light pulses i.e. their data word, for the short time interval during which the key is depressed or for a pre-determined time. As a rule, a binary data word consists of the pulses 0 and 1, wherein a 0 is usually a short pulse and a 1 is a long pulse or two successive pulses. In order to obtain a greater range, an appropriately high current is sent through the transmission elements for a short time, these elements usually being light emitting diodes. The pulse length usually amounts to just a few μs so that a relatively high average current is reached during the transmit mode. In practice, this current ranges from just a few up to several hundred milliamperes for a supply voltage of e.g. 3 V during the data transmission process. These devices are therefore provided with a “powerful” battery and are only suitable for sporadic operation. Continuous operation would discharge the battery within a short time.
An optical data transmission device for a device comprising a receiver which is arranged at the location of the device and also comprises a plurality of portable transmitters which are carried by a corresponding plurality of persons is known from EP 1 229 672 B1. This data transmission device consists of a movement detector which is arranged at the location, and a plurality of optical wireless data links between the receiver and a plurality of transmitters, wherein each transmitter has a unique identification code and also means for the actuation of an alarm. The alarm is set off if there is detected by the movement detector a person at the location who does not have a transmitter having an identification code which indicates that access to the location may be granted to that person. For a 21 bit data word here, there is a transmitting time of 288 μs with a repetition rate of a minimum of two seconds. Experience shows that in the case of long data words and thus long transmitting times of this type, more energy must be made available for the transmission of the information. In addition, means for identifying the signal are necessary in the event of collision of data as a result of a signal superimposition. This can easily happen due to overlap of the temporally relatively long data words.
EP 1 804 220 A1 illustrates a method for the authorized grant of a service and portable equipment for carrying out this function without using an optical data transmission system. The communication link between the portable equipment and the medium is intended for short range operation. The portable equipment transmits one or more identifiers over several different communication links so as to gain access to the service. The employment of the short range communication link does not enable the operator to pass through a door without breaking his stride. When using the service, the operator must move towards the medium with his portable equipment.
From DE 10 2005 062 632 A1, there is known an automatic administration system for persons staying within the confines of a building wherein the access authorization is programmed into an identification element at the entrance of the building. Individual access checking devices then check this information. This is preferably effected by providing the identification elements with optically, acoustically, magnetically and/or electronically detectable, customisable identifiers. A transponder is used, above all, in the case of an electronically detectable identifier system wherein a fixed transmit/receiver device sends out a signal which induces the transponder or the corresponding transmit/receiver device of the identification element to send out a signal comprising the individualized identifier. This signal is then detected on the part of the fixed device for the purposes of controlling the entrance. If one is working with optical means, then an identifier is merely imprinted on the identification element which is then read out locally from the identification element. Hereby, optical transmission e.g. by light radiation does not take place.